I compared the decorated version with the one starting LordsAWar with the undecorated windows, then switch to decorated windows and proceed. Result: there are only a few elements differing (like e.g. the menu, some buttons).

I also compared the undecorated version with the one starting LordsAWar with the decorated windows, then switch to undecorated windows and proceed. Result: see above.

Perhaps these elements are buggy.

Number of appearings of the "bunch of warnings":
splash: 1
new game: 2
load game: 0
load scenario:1
new network: 0
new pbm: 2
preferences: 1

It only happens, when using the decorated windows set. I think these error messages are related to bug #24425. I realized, that switching to decorated windows works without restart, but not properly. The warnings appear also after a restart.

The warnings show, that there's still something wrong with the decorated windows, which isn't solved actually despite the "restart hack".